Conventionally, a type of walk assistance device is known including a trunk attachment portion attached to a user's trunk, a thigh attachment portion attached to a thigh of a user's leg, the thigh attachment portion being connected to the trunk attachment portion via a hip joint portion corresponding to a human hip joint, a crus attachment portion attached to the crus of the user's leg, the crus attachment portion being connected to the thigh attachment portion via a knee joint portion corresponding to a human knee joint, and a foot attachment portion attached to a user's foot, the foot attachment portion being connected to the crus attachment portion via an ankle joint portion corresponding to a human ankle joint. Driving sources which drive the joint portions are provided coaxially with the joint portions, respectively (refer to Japanese Patent Laid-Open No. 2003-220102, for example). This type of walk assistance device is capable of assisting the user in walking by applying an assist moment from the driving source for the hip joint portion to the user's thigh via the thigh attachment portion, applying an assist moment from the driving source for the knee joint portion to the user's crus via the crus attachment portion, and applying an assist moment from the driving source for the ankle joint portion to the user's foot via the foot attachment portion.
Although the above conventional walk assistance device is capable of assisting all movements of the user's thigh, crus, and foot, the thigh and the crus are restrained by the thigh attachment portion and the crus attachment portion and therefore the user has an extremely constrained feeling. In addition, it is necessary to provide the driving sources for the joint portions such as the hip joint portion, the knee joint portion, and the ankle joint portion, respectively, which increases the cost disadvantageously.
In order to solve the above problem, it is conceivable to adapt the walk assistance device to include a load transmit portion, a foot attachment portion attached to the user's foot, and a leg link between the load transmit portion and the foot attachment portion, wherein a force generated by the leg link is transmitted to the user's trunk via the load transmit portion. According thereto, the walk assistance device can assist walking by reducing the load on the user's leg by means of the force from the leg link transmitted to the user's trunk via the load transmit portion. Furthermore, it is possible to reduce the constrained feeling by making the leg link free from the user's leg. In this condition, if the leg link includes an upper first link portion which is connected to the load transmit portion via a first joint portion, a lower second link portion which is connected to the foot attachment portion via a second joint portion, a middle third joint portion which connects the first link portion to the second link portion in such a way that a distance between the first joint portion and the second joint portion is variable, and a driving source which drives the third joint portion, then the load on the user's leg can be reduced by generating a force in the direction of extending the distance between the first joint portion and the second joint portion at the third joint portion by means of the driving source.
Note that the leg link is freed from the user's leg while the leg link on the side of the user's free leg (the leg whose foot is leaving the ground) swings forward with the first joint portion as a fulcrum, following the forward swing of the free leg. Therefore, in the case of increase in the moment of inertia around the first joint portion of the leg link, the user feels heavy in the leg in walking due to the moment of inertia of the leg link applied to the user's leg.